Conventionally, power devices using silicon (Si) semiconductors have been used as devices for power electronics. Devices for power electronics are desired to operate at a higher frequency with a larger current. Various studies for research and development have been made to improve the performance of silicon power devices.
However, the performance of the silicon power devices is now approaching the theoretical limit thereof. In addition, power devices are occasionally required to operate in severe environments, for example, at a high temperature or under radiation. Silicon semiconductors are not suitable to use in such severe environments. For these reasons, studies are being made regarding devices using semiconductor materials other than silicon.
Among various semiconductor materials, a silicon carbide (SiC) semiconductor has a large forbidden band width (3.26 eV in the case of type 4H) and is superb in electric conduction control and radiation resistance at high temperature. The silicon carbide semiconductor has a breakdown field which is about one digit higher than that of silicon and also has a saturation drift speed of electrons which is about twice as high as that of silicon, and so has a high withstand voltage and is controllable at a high frequency with a large power. Owing to these physical properties thereof as a semiconductor material, silicon carbide is anticipated as a semiconductor material for power devices operable at a higher frequency with a larger current.
For forming a device such as a MISFET or the like using silicon carbide, nickel (Ni) is widely used as an ohmic electrode material for n-type silicon carbide. However, where nickel is merely deposited on n-type silicon carbide by vacuum vapor deposition or the like, the interface between nickel and n-type silicon carbide exhibits a rectifying function but does not exhibit any ohmic function because a Schottky barrier is formed at the interface between the metal and the semiconductor. Usually, an ohmic electrode can only be obtained by performing thermal treatment on nickel, after being deposited, at a high temperature to promote diffusion of nickel into silicon carbide and diffusion of silicon in the silicon carbide into nickel. Semiconductor devices including an ohmic electrode formed of nickel provided on an n-type silicon carbide semiconductor are disclosed in, for example, Patent Documents 1 and 2.
Patent Document 1: Japanese Laid-Open Patent Publication No. 7-99169
Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-243323